Electrons and Electron Beams

When a plastic ruler is placed near some very tiny pieces of paper, it will not have any effect on them. However, if the ruler is rubbed with a duster, it is found that the pieces of paper are attracted to the ruler. This difference in behaviour is explained by saying that the ruler has become electrically charged when rubbed with the duster. Because the ruler is charged, it exerts an electric force on the paper; the nearer the ruler is to the paper the greater the force it exerts. Charging the ruler by rubbing it with a duster is called charging by friction.